Start port forwards on the phone, listed in a sheet off its terminal

The Android head skipped a host's forwards because it had nowhere to show
what was listening. Its terminal bar now draws a ⇄ for a tab that has
forwards, carrying the listening count in green and turning the danger
colour when one failed, and it raises a sheet over the same rows the
desktop sidebar lists: notation, route, state, failure reason and a tap
to switch each off or on. MainWindowViewModel.StartForwards no longer
returns early on Android.

The sheet is shell state like the connect sheet: it collapses the
renderer while up, is lowered by leaving the terminal, and the back
gesture closes it first.

Forwards ride the shell's keep-alive rather than holding the foreground
service themselves, since a forward cannot outlive its shell. They only
change the notification, which now counts ports forwarded beside shells
connected, fed by a new ForwardsChanged event and ListeningForwardCount.

The phone still has no editor for forwards; the More screen card, README
and design-import-gaps now say they run here and are edited on the
desktop.

src/DodoSSH.Client.Android/Views/PhoneShell.axaml.cs

@@ -579,7 +579,7 @@ internal sealed partial class PhoneShell : UserControl
     /// <para>
     /// <b>The connect menu is a second such guard, and it matters more than the first.</b> A terminal now
     /// fills the screen — no header, no bottom bar — so while that menu is up this gesture is the only way
-    /// off it other than the scrim and CANCEL. It is checked before the terminal is dismissed for the
+    /// off it other than the scrim and CANCEL. The forwards sheet, raised from the same bar, shares the guard. It is checked before the terminal is dismissed for the
     /// reason it is drawn over it: back takes the topmost thing, and dismissing the surface underneath a
     /// menu would take two, neither of them the one being looked at.
     /// </para>
@@ -598,13 +598,12 @@ internal sealed partial class PhoneShell : UserControl
             return;
         }
 
-        // The connect menu, which is raised from the terminal's own bar and is the topmost thing the phone
-        // draws while it is up. Ahead of the editors below because it is nearer, and ahead of leaving the
-        // terminal because a gesture that dismissed the surface underneath a menu would close two things at
-        // once — and the one the user was looking at would not be either of them.
-        if (current.IsConnectSheetOpen)
+        // The connect menu or the forwards sheet, which are raised from the terminal's own bar and are the
+        // topmost thing the phone draws while one is up. Ahead of the editors below because they are nearer,
+        // and ahead of leaving the terminal because a gesture that dismissed the surface underneath a sheet
+        // would close two things at once — and the one the user was looking at would not be either of them.
+        if (TryCloseATerminalSheet(current))
         {
-            current.CloseConnectSheetCommand.Execute(null);
             e.Handled = true;
             return;
         }
@@ -647,6 +646,31 @@ internal sealed partial class PhoneShell : UserControl
         }
     }
 
+    /// <summary>
+    /// Lowers the connect menu or the forwards sheet, whichever is up over the terminal.
+    /// </summary>
+    /// <returns>Whether one was closed, and so whether back has been spent.</returns>
+    /// <remarks>
+    /// Both are raised from the terminal's bar and the scrim under each covers that bar, so at most one is
+    /// ever open; the order below decides nothing.
+    /// </remarks>
+    private static bool TryCloseATerminalSheet(MainWindowViewModel current)
+    {
+        if (current.IsConnectSheetOpen)
+        {
+            current.CloseConnectSheetCommand.Execute(null);
+            return true;
+        }
+
+        if (current.IsForwardsSheetOpen)
+        {
+            current.CloseForwardsSheetCommand.Execute(null);
+            return true;
+        }
+
+        return false;
+    }
+
     /// <summary>
     /// Lowers whatever the hosts screen has raised over its list, topmost first.
     /// </summary>